The invention relates to a planetary transmission employing planetary gear assemblies which incorporate discs. More specifically, the invention relates to a novel planetary gear assembly and a planetary transmission employing the novel gear assembly.
A planetary transmission may be used to provide rotation of an output shaft which is different from the rotation of an input shaft. One type of planetary transmission employs rigid and precisely engaged gears, but costs for this type of transmission are relatively high. In another type of planetary transmission, loosely engaged gears are employed, but their use is accompanied by considerable undesirable noise and vibration and concomitant low efficiency of power transmission.
Designs for planetary transmissions employing loosely engaged teeth have been developed in an effort to reduce noise and vibration. In one design, discs are placed on both sides of the planetary gear and on the same shaft as the planetary gear.
U.S. Pat. No. 3,293,928, published Dec. 27, 1966, discloses a torque convertor which includes planetary gear assemblies having a planetary ring gear sandwiched between two planetary discs. The disc diameters are equal to the diameter of the pitch circle of the gears.
U.S. Pat. No. 3,548,679 published Dec. 22, 1970, describes a toothed gear system and a bevel gear system with friction gears whose diameters are equal to those of the pitch circle on the side of each gear. The friction gears transmit power radially and eliminate backlash.
Japanese Model Publication No. 16,918/55 describes a planetary gear system in which extreme accuracy in fitting parts together is not necessary. A roller is set on one side of the planetary gear, and a cylindrical pipe is set on side of the ring gear. The roller and pipe have diameters equal to the pitch circles of the gears. The rollers and pipes eliminate problems resulting from inaccuracies in size. The rollers are mounted on the same shaft as the planetary gears but apart from them. The rollers and the pipes receives radial pressure and provide smooth revolution. However, because the rollers are mounted on the same shaft as the planetary gears and set apart from them, the shafts must be long making the entire system very bulky.
In Japanese Utility Model Publication No. 25,692/69 another technique is disclosed for overcoming disadvantages in planetary gear systems. Here, the planetary gears are made of elastic material, namely rubber. Elastic gears, however, impose limitations on the transmission. The torque allowable in an elastic gear system is relatively low, and continuous expansion and contraction of rubber parts produce heat and accelerate the deterioration of the elastic material.
Another method for overcoming difficulties in planetary transmissions is disclosed in Japanese Utility Model Publication No. 17,588/60 and Japanese Patent Publication No. 22,661/61. An intermediate ring is placed between a planetary gear ring and the planetary shaft, and a clearance is provided between the intermediate ring and the planetary gear ring so that the planetary gear ring can slightly change its relative position in relation to the planetary shaft and compensate for inaccuracies in gear dimensions. In this type of device, pitch discs or pitch rings are not necessary. A difficulty associated with the transmission of this type is that transmitted power is not evenly distributed to the gear teeth. If the sun gear or the carrier were movable radially, the center of the shaft would move to compensate for inaccuracies in engagement. However, in many instances, this is not the case. Therefore, a transmission of this type does not average transmission torque. Because of this disadvantage, this technique is even less suitable than the elastic gear device.
In Japanese Patent Publication No. 17,111/79 published June 27, 1979, a planetary transmission is disclosed having features employing both the pitch disc technique and the elastic gear method. A planetary gear is placed loosely between two pitch discs rotatably mounted on planetary gear shafts supported by a carrier plate fixed on an output shaft. The outer diameters of the pitch discs are equal to those of the pitch circles of both the planetary gear and the outer pitch rings fixed to a case. The outer diameter of the pitch discs is also equal to that of the pitch circle of the ring gear. The pitch discs of the planetary gears contact and roll on the pitch rings of the outer, inner-toothed gear. Because of rolling contact, the contact between the disc and ring transmit no torque. But the contact transmits pressure face-to-face.
A planetary transmission employing discs having pitch circle diameters is disclosed in copending U.S. application Ser. No. 513,207, filed July 12, 1983 by the present inventor.
In a planetary transmission, the sun gear and the carrier are connected to input and output shafts in order to transmit rotation. However, the planetary gears are freely turning wheels which have neither input nor output shafts nor corresponding shaft bearings. Therefore the planetary gears can move in a radial direction during operation. When planetary gears deviate from their ordinary position during operation, they often engage other gears with an abnormally strong force. Such rough engagement brings about excessive noise and wear and tear on the gears.
Another source of abnormal wear and tear on the gears and abnormally loud noise during operation is when the teeth of one gear tend to adhere to the teeth of another gear. When this teeth interference occurs, an excessively strong force is required to rotate the interfering gears.
For these reasons planetary gear assemblies tend to generate excessive noise and tend to operate at unexpectedly low torque transmission efficiency.
If the accuracy of finishing the gear teeth is raised, the pressures at all tooth engagement points should become equivalent, and the gears and carrier should rotate smoothly and with even force. However, slight inaccuracies between gears is inevitable, and gears cannot compensate for the inequalities of pressures.
One way to improve the accuracy of gear finishing in order to avoid the inequality of gear pressure would be to control the limit of inaccuracy to be less than several microns. However, such a technique would be very expensive because the cost of grinding a tooth is considerably high, and the whole cost of grinding an entire gear is proportional to the number of teeth.
In the pitch circle disc method referred to above, the pitch discs of the planetary gears contact and roll on the pitch rings of the outer, inner-toothed gear. Because of rolling contact, the contact between the disc and the ring transmit no torque, but the rolling contact does transmit pressure face-to-face. Deep teeth interference is prevented by the contact between the pitch circle disc and the pitch circle ring.
Yet another apparatus designed to eliminate pressure differences at gear engagement points is a planetary gear assembly disclosed in copending U.S. patent application Ser. No. 523,919 filed Aug. 16, 1983 by the present inventor in which a planetary tranmission is disclosed having planetary ring gears sandwiched between front and back thrust discs and having an outer, inner-toothed annular ring gear sandwiched between front and back thrust rings. The thrust disc outer diameter is equal to or preferably greater than the planetary ring gear tooth-tip circle diameter. The thrust ring inner diameter is equal to or preferably greater than the tooth-bottom circle diameter of the outer, inner-toothed ring gear.
When a planetary gear with a pitch-circle disc is seen for a side view, half the gear teeth stubs are visible; that is, one module of teeth project outward from the disc. When a planetary gear with a tooth-edge-circle disc is seen from a side view, no portion of the gear teeth appears.
The planetary gear assembly having tooth-edge-discs have several advantages over a planetary gear assembly having pitch-circle discs. One advantage of the tooth-edge-disc assembly is that it is strongly resistant to axial thrust during operation of the transmission. Thrust is generated by a relative axial displacement of the sun gear shaft and the carrier shaft during operation of the transmission.
In the pitch-circle-disc type of planetary gear assembly, only half of the gear teeth tops, namely one module of teeth, are held between adjacent gears. Under excessive thrust, the gear teeth tops interfere with one another. In contrast, with the tooth-edge-disc planetary gear assembly described in U.S. Ser. No. 523,919, the entire length of the gear tooth tops, namely more than two modules, are sandwiched between two discs. Because a total area of the teeth sides is protected by thrust discs and because the total area of the teeth sides for sustaining thrust is relatively large, even if continuous thrust acts on the gears, the gears are protected from defacement and transmission efficiency does not decrease.
The tooth-edge-disc planetary gear assembly is comprised of fewer components than the pitch-circle-disc type of planetary gear assembly. This aspect is very important because an assembly having fewer parts to assemble is easier to mass-produce.
To manufacture a pitch-circle-disc planetary gear assembly, three or four planetary gears are supported by planetary shafts fixed on one carrier plate along with a sun gear and an outer, inner-toothed ring gear Another carrier plate is fixed to the planetary shafts, and these components are made up into a planetary transmission. Two outer pitch-circle rings are separate components. These outer pitch circle rings are mounted on the inner-toothed ring gear by screws for installing the assembly into a casing. Thus, with the pitch-circle-disc type of planetary transmission, both the planetary gear assembly and the outer, inner-toothed ring gear assembly require three components. No single component is satisfactory for producing an individual gear.
Many efforts have been made to solve the problems of teeth interference in planetary gear transmissions. In efforts to make improvements thus far, the components focused upon for improvement have been the outer, inner-toothed ring gear and the planetary gear assemblies. Improvements are easily brought about upon planetary gears because they are freely rotating, do not transmit torque, and are not attached to shafts. To solve the problem of the discrepancy of forces acting on a plurality of planetary gears, improvements in the structure of the planetary gears has been most effective
Generally, a planetary gear assembly and an outer, inner-toothed ring gear are fixed to a casing, and a sun gear or a carrier is connected to either an input shaft or an output shaft. When used for deceleration, the sun gear is attached to an input shaft. For use in acceleration, the carrier is attached to the input shaft.
The center lines of the input shaft and the output shaft should lie along the same axial line. In addition, the center line of the outer, inner-toothed ring gear must also coincide with the center line of the input and output shafts. These are general requirements for mounting a planetary gear transmission assembly in a casing.
In many cases an input shaft is connected to a motor at one end and is supported by a bearing at an intermediate location. The output shaft is connected with a load at one end and supported by one or more bearings at an intermediate location.
However, the casing for securing the assembly is not always manufactured with high accuracy. In the instance of a plastic casing, considerable errors may occur in the sizes of the molded casings.
When the center lines of the input shaft, the output shaft, and the outer, inner-toothed ring gear do not coincide, the sun gear and the carrier vibrate in a radial direction, and the carrier inclines out of normal position. In such a case, significant teeth interference occurs between the sun gear and the planetary gears. The teeth interference generates considerable noise and significantly reduces transmission operating efficiency.
It is well known in the prior art that gears which have side discs are protected from radial forces. However, the prior art discloses pitch-circle discs only. Thus, a sun gear or an outer, inner-toothed ring gear, in accordance with the prior art, must be composed of three components in order to have pitch-circle discs on both sides.
Furthermore, since both the sun gear and the outer, inner-toothed ring gear transmit torque, unlike toothed ring gear has side discs, then the side discs should be fixed upon the gear part by suitable securing devices such as screws, etc.
Accordingly, it is a primary object of the present invention to provide a planetary transmission having planetary gear assemblies with planetary discs and having a unitary sun gear and a unitary outer, inner-toothed ring gear which engage the planetary discs.
An advantage of the present invention is the provision of a planetary gear transmission in which the number of components is fewer and the assembly cost is less expensive than a transmission of the pitch-circle-disc type.
Another advantage of the present invention is the provision of a planetary gear transmission in which all the gears have either tooth-edge-circle discs or tooth-root-circle discs on both sides for protecting the gears from radial forces.
Another advantage of the present invention is the provision of a planetary gear assembly in which the planetary discs prevent teeth interference between meshing gears.
Another advantage of the present invention is the provision of a planetary gear assembly having planetary discs in which the transmission efficiency is higher than a planetary gear assembly not having planetary discs.